The sources of supply consist of groundwater or aquifers and surface waters, streams and lakes. The first two sources of supply generally supply water which in its natural state is suitable for use.
Surface waters sometimes need, before being allowed to use, corrective treatments of the natural characteristics, in any case necessary for the potable use of the resource. When the water table flows through an unsaturated porous layer supported by an impermeable layer, there is a free surface or water table; when the permeable layer is contained between two impermeable layers, two cases can occur: if the permeable zone is not saturated the groundwater is free and deep while, if the permeable zone is saturated and subject to pressure such that the piezometric levels are above the surface at the bottom of the upper layer, there is a pressure layer or an artesian layer.
Figure 1 – Types of pitches
When groundwater reaches the surface of the soil, they give rise to natural springs called springs which, with respect to topographical and geological situations, can be classified into the following types.
Bottom sources
They originate from the outcrop of the impermeable layer that constitutes the bottom surface (Figure 2).
Figure 2 – Background sources
From detritus
The bottom surface, impermeable, is covered by a detrital mass (dejection cone, moraine, landslide materials) which is the seat of the aquifer which emerges, downstream, at the foot of the debris.
Monoclinal or fluvial
The bottom surface, which has a constant direction and uniform slope (monocline), emerges on a slope.
Syncline or lacual
The impermeable layer has an upward (synclinal) concavity outcropping on a slope.
Sources of surfacing or emergence
The ground cuts locally, by incision, the surface of the water table, generating the sources of the slope or by depression. In this case, two springs may occur on opposite slopes with the outcrop of valley springs (Figure 3).
Figure 3 – Sources of outcrop or emergence
Sources of drainage
They result from the existence, within a permeable mass, of cracks which constitute a vascular water circulation system. They are typical of fractured vehicles and in karst areas.
Touch sources
They are generated by the outcrop of a sub-vertical waterproof layer, generally not supporting the aquifer.
Artesian springs
They are fed by pressurized aquifers in the presence of fractures of the impermeable layer or fault with rejection of the surface state (Figure 4).
Figure 4 – Artesian sources
